Exotic Form of Silicon Improves Solar Cells

DAVIS, Calif., Jan. 30, 2013 — An exotic form of silicon may improve solar cell efficiency by as much as 70 percent, according to computer simulations conducted by an international team of scientists.

Solar cells are based on the photoelectric effect: a photon hits a silicon crystal and generates a negatively charged electron and a positively charged hole. Collecting those electron-hole pairs generates electric current.

Conventional photovoltaics generate one electron-hole pair per incoming photon, and theoretically have a maximum efficiency of 33 percent. But now, researchers at the University of California, Davis, and in Hungary have taken this idea a step further to generate more than one electron-hole pair per photon. To achieve an improved efficiency, the team simulated the behavior of a structure of silicon called silicon BC8, which is formed under high pressure but is stable at normal pressures.

“This approach is capable of increasing the maximum efficiency to 42 percent, beyond any solar cell available today, which would be a pretty big deal,” said Stefan Wippermann, a postdoctoral researcher at UC Davis. “In fact, there is reason to believe that if parabolic mirrors are used to focus the sunlight on such a new-paradigm solar cell, its efficiency could reach as high as 70 percent.”

Computer simulations performed at the University of California, Davis, show that when a light particle (blue wave on left) hits a crystal of a high-pressure form of silicon, it releases two electron-hole pairs (red circles/green rings), which generate electric current. Courtesy of Stefan Wippermann/UC Davis.
Because of their small size, nanoparticles have different properties than bulk materials. In particular, their probability of generating more than one electron-hole pair is significantly enhanced because of an effect called quantum confinement.

“But with nanoparticles of conventional silicon, the paradigm works only in ultraviolet light,” Wippermann said. “This new approach will become useful only when it is demonstrated to work in visible sunlight.”

Computer simulations run through the National Energy Research Scientific Supercomputing Center at the Lawrence Berkeley Laboratory demonstrated that nanoparticles of silicon BC8 indeed generate multiple electron-hole pairs per photon even when exposed to visible light.

“This is more than an academic exercise,” said UC Davis professor of physics
Gergely Zimanyi. “A Harvard-MIT paper showed that when normal silicon solar cells are irradiated with laser light, the energy the laser emits may create a local pressure high enough to form BC8 nanocrystals. Thus, laser or chemical pressure treatment of existing solar cells may turn them into these higher-efficiency cells.”